Remove some dead code

* The following modules are no longer used and have been removed completely:

	GF.Compile.Compute.ConcreteLazy
	GF.Compile.Compute.ConcreteStrict
	GF.Compile.Refresh

* The STM monad has been commented out. It was only used in
  GF.Compile.SubExpOpt, where could be replaced with a plain State monad,
  since no error handling was needed. One of the functions was hardwired to
  the Err monad, but did in fact not use error handling, so it was turned
  into a pure function.

* The function errVal has been renamed to fromErr (since it is analogous to
  fromMaybe).

* Replaced 'fail' with 'raise' and 'return ()' with 'done' in a few places.

* Some additional old code that was already commented out has been removed.

src/compiler/GF/Data/ErrM.hs

@@ -12,15 +12,25 @@
 -- hack for BNFC generated files. AR 21/9/2003
 -----------------------------------------------------------------------------
 
-module GF.Data.ErrM (Err(..)) where
+module GF.Data.ErrM where
 
 import Control.Monad (MonadPlus(..),ap)
 import Control.Applicative
 
--- | like @Maybe@ type with error msgs
+-- | Like 'Maybe' type with error msgs
 data Err a = Ok a | Bad String
   deriving (Read, Show, Eq)
 
+-- | Analogue of 'maybe'
+err :: (String -> b) -> (a -> b) -> Err a -> b 
+err d f e = case e of
+  Ok a -> f a
+  Bad s -> d s
+
+-- | Analogue of 'fromMaybe'
+fromErr :: a -> Err a -> a
+fromErr a = err (const a) id
+
 instance Monad Err where
   return      = Ok
   fail        = Bad